1. Field of the Invention
The present invention relates in general to a control apparatus for a vehicular power transmitting system including an electrically controlled differential portion operable to distribute an output of a drive power source to a first electric motor and a power transmitting member, and a second electric motor connected to a power transmitting path between an output shaft of the drive power source and a drive wheel of a vehicle, and more particularly to techniques for optimizing a balance between an amount of charging of an electric-energy storage device with an electric energy generated by the first electric motor and an amount of electric energy of the electric-energy storage device consumed by the second electric motor.
2. Discussion of Prior Art
There is known a vehicular power transmitting system including (a) an electrically controlled differential portion operable to distribute an output of a drive power source of a vehicle to a first electric motor and a power transmitting member, (b) a second electric motor connected to a power transmitting path between an output shaft of the drive power source and drive wheels of the vehicle, (c) a coupling device provided in the electrically controlled differential portion and operable to selectively switch the differential portion between a differential state and a non-differential state, and (d) an electric-energy storage device operable to store and discharge an electric energy supplied from the first or second electric motor. JP-2005-331063A discloses an example of such a vehicular power transmitting system in the form of a hybrid vehicle power transmitting system. In the hybrid vehicle power transmitting system disclosed in this publication, the electrically controlled differential portion is constituted by a planetary gear set, for example, and this differential portion performs a differential function so that a major part of a drive force generated by the drive power source is mechanically transmitted to the vehicle drive wheels, while a remaining part of the drive force is converted by the first electric motor into an electric energy, which is stored in the electric-energy storage device or supplied to the second electric motor, through an inverter. The vehicular power transmitting system of the publication is further configured to permit starting of the vehicle with the drive power source by placing the coupling device in a slipping state (partially engaged state), even where the first electric motor is not able to generate a sufficient reaction torque during the vehicle starting with the drive power source.
In the power transmitting system for a hybrid vehicle as disclosed in the above-identified publication, the electric-energy storage device which is charged and discharged by the first and second electric motors can be charged with an electric energy generated by the first electric motor when the amount of electric energy stored in the electric-energy storage device is relatively small, but may be excessively charged if the electric energy generated by the first electric motor is supplied to the electric-energy storage device when the amount of electric energy stored in the electric-energy storage device is relatively large. It is possible to prevent overcharging of the electric-energy storage device by limiting the amount of electric energy generated by the first electric motor. However, the limitation of the electric energy generated by the first electric motor results in a failure of the first electric motor to generate a sufficient reaction torque, and consequent limitation of an output torque to be transmitted to the drive wheels. The above-identified publication proposes to place the coupling device in the slipping state for permitting mechanical generation of a reaction torque. However, the amount of slipping of the coupling device should be rapidly increased if the slipping control of the coupling device is implemented immediately before the electric-energy storage device is excessively charged. The rapid increase of the slipping amount may cause an overloading of the coupling device.